Low temperature, low-resistivity heavily doped p-type polysilicon deposition

ABSTRACT

A method to create a low resistivity P+ in-situ doped polysilicon film at low temperature from SiH 4  and BCl 3  with no anneal required. At conventional dopant concentrations using these source gases, as deposition temperature decreases below about 550 degrees C., deposition rate decreases and sheet resistance increases, making production of a high-quality film impossible. By flowing very high amounts of BCl 3 , however, such that the concentration of boron atoms in the resultant film is about 7×10 20  or higher, the deposition rate and sheet resistance are improved, and a high-quality film is produced.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method of depositing dopedpolycrystalline silicon, hereinafter referred to as polysilicon, at lowtemperature.

[0002] Thin doped polysilicon films are typically deposited attemperatures between 540 and 625 degrees C. by low pressure chemicalvapor deposition (LPCVD.) Alternatively, an amorphous silicon film canbe deposited at lower temperatures with dopants, then annealed afterdeposition at temperatures of 550 degrees C. or greater to crystallizethe silicon and activate the dopants. In general, as depositiontemperature drops, the deposition rate and quality of doped polysiliconfilms decreases.

[0003] The temperatures required to create doped polysilicon films usingconventional methods are incompatible with other processes and materialsthat may be desirable. For example, aluminum metallization withstands amaximum practical fabrication temperature of only 475 degrees C. forsemiconductor processing.

[0004] Ishihara, U.S. Pat. No. 5,956,602, “Deposition of Polycrystal SiFilm,” discloses a method to deposit a doped polysilicon film attemperatures below 500 degrees C. This method introduces a source gaslike SiH₄ and a dopant gas such as BCl₃, PH₃, or Al(CH₃)₃. The two gasesare flowed at different times, without overlapping. Hydrogen plasma isthen used to anneal the film. The process is repeated until a film ofthe desired thickness is produced.

[0005] While this method claims to produce doped polysilicon attemperatures below 500 degrees C., it has disadvantages. Repeatedlyintroducing the source and dopant gases at different times and annealingwith hydrogen plasma involves significant process complexity andrequires specialized equipment. Further, alternately introducing thesilicon and dopant source gases may result in dopant nonuniformitiesthroughout the film.

[0006] There is a need, therefore, to create high-quality dopedpolysilicon films at temperatures less than those used in conventionalprocesses and that do not require a subsequent high temperature anneal.It would be preferable if complex deposition cycles and specializedequipment are not required.

SUMMARY OF THE INVENTION

[0007] The present invention is defined by the following claims, andnothing in this section should be taken as a limitation on those claims.In general, the invention is directed to a method to depositlow-resistivity doped polysilicon at low temperatures.

[0008] According to one aspect of the invention, a method for depositinga doped polysilicon film comprises providing a surface and substantiallysimultaneously flowing SiH₄ and BCl₃ over the surface at a temperatureless than or equal to about 500 degrees Celsius under conditions thatachieve an average concentration in the doped polysilicon film ofbetween about 7×10²⁰ and about 3×10²¹ boron atoms per cubic centimeter.

[0009] According to another aspect of the invention, a method forforming in-situ doped polysilicon comprises providing a surface andsubstantially simultaneously flowing a first source gas comprising SiH₄and a second source gas comprising BCl₃ over the surface at atemperature less than about 500 degrees Celsius under conditionssufficient to achieve in the doped polysilicon an average concentrationof between about 7×10²⁰ and about 3×10²¹ boron atoms per cubiccentimeter.

[0010] A related embodiment provides for a semiconductor devicecomprising in-situ doped polysilicon formed by a method comprisingproviding a surface, and substantially simultaneously flowing SiH₄ andBCl₃ over the surface at a temperature less than about 500 degreesCelsius, wherein an average concentration of boron atoms in thepolysilicon of between about 7×10²⁰ and about 3×10²¹ per cubiccentimeter is achieved.

[0011] Another related embodiment provides for a monolithic threedimensional memory comprising polysilicon formed by a method comprisingsubstantially simultaneously flowing SiH₄ and BCl₃ at a temperature lessthan or equal to about 500 degrees Celsius, wherein an averageconcentration of boron atoms in the polysilicon is between about 7×10²⁰and about 3×10²¹ per cubic centimeter, wherein the monolithic threedimensional memory comprises two or more memory levels.

[0012] A preferred embodiment provides for a method for depositingin-situ doped polysilicon, the method comprising providing a surfacecomprising a substantially horizontal surface and a substantiallyvertical sidewall descending from the horizontal surface, the sidewallhaving a top, and depositing an in-situ doped polysilicon film on thesurface at a temperature less than about 500 degrees Celsius, wherein afirst thickness of the film at its thinnest point on the verticalsidewall is at least 80 percent of a second thickness of the film on thesidewall at the top of the sidewall, and a third thickness of the filmon the horizontal surface is at least 200 angstroms.

[0013] Another preferred embodiment provides for an in-situ dopedpolysilicon film wherein the polysilicon film was deposited at atemperature less than about 500 degrees Celsius, and the polysiliconfilm is deposited by substantially simultaneously flowing a first sourcegas comprising SiH₄ and a second source gas comprising BCl₃, wherein thepolysilicon film has a sheet resistance less than about 280 ohms/square.

[0014] Other preferred embodiments are provided, and each of thepreferred embodiments can be used alone or in combination with oneanother.

[0015] The preferred embodiments will now be described with reference tothe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIGS. 1a through 1 c illustrate decreasing quality of P+polysilicon film at progressively lower deposition temperatures.

[0017]FIG. 2 is a graph showing sheet resistance of polysilicon filmsvs. deposition temperature for different dopant concentrations.

[0018]FIG. 3 is a graph showing deposition rate in angstroms per minutevs. deposition temperature for different dopant concentrations.

[0019]FIGS. 4a through 4 c illustrate film step coverage over unevensurfaces.

DETAILED DESCRIPTION OF THE INVENTION

[0020] A challenge of semiconductor device design is to select materialsthat are thermally compatible. If a deposition or anneal processrequires high temperatures, other materials already present must becompatible with those temperatures.

[0021] Doped polysilicon has many uses in the semiconductor industry.For many designs and devices, it would be advantageous to deposit alow-resistance doped polysilicon film with good step coverage at lowtemperature. Specifically, it would be useful to form a low-resistancedoped polysilicon film with good step coverage at temperatures belowabout 500 degrees. Aluminum wiring is used in many integrated circuitdesigns, but when used with doped polysilicon, the aluminum can only bedeposited after the doped polysilicon is deposited and in a lowresistivity state, since aluminum will soften and/or extrude attemperatures above about 475 degrees C., making practical semiconductorfabrication difficult. Thus even lower temperatures for deposition ofdoped polysilicon, for example about 475 degrees and below, would alsohighly advantageous. By employing methods according to the presentinvention, aluminum wiring, for example, could be formed before dopedpolysilicon deposition, then low-resistance doped polysilicon formedlater, at temperatures low enough to avoid damaging the aluminumstructures.

[0022] At about 550 degrees C., heavily doped p-type (P+) depositedsilicon doped in situ (by providing dopant atoms during deposition,rather than by implanting them later) is polycrystalline and conductiveand has low resistivity. This is in contrast to heavily doped n-typesilicon, which tends to be amorphous at this deposition temperature, andmust be annealed at a temperature of, for example, about 550 degrees C.or greater to crystallize the silicon and render it conductive.

[0023] As temperature falls below 550 degrees C., however, the rate ofdeposition of P+ polysilicon generally decreases and the quality of thefilm begins to degrade. FIG. 1a is a scanning electron microscope imageof a high-quality P+ polysilicon film 200 nm thick deposited by flowingSiH₄, BCl₃ and helium at 550 degrees C. In FIG. 1b, at a temperature of500 degrees C., it can be seen a 200 nm thick film is rougher; this filmwill have higher resistivity. FIG. 1c shows a 200 nm thick P+polysilicon film deposited at 460 degrees C. This film is very rough anddiscontinuous and would not be suitable for uses requiring a conductivefilm. In FIGS. 1a, 1 b, and 1 c, the concentration of boron atoms (ap-type dopant) in the polysilicon film is 2×10²⁰ atoms/cm³, a fairlytypical dopant concentration for heavily doped polysilicon.

[0024]FIG. 2 shows sheet resistance for P+ in-situ doped polysiliconfilms deposited at different temperatures. It will be seen on curve Athat for boron concentrations of 2×10²⁰ atoms/cm³, sheet resistancegenerally increases as deposition temperature decreases. In someapplications the lowest possible sheet resistance is advantageous fordevice performance.

[0025] Deposition of P+ polysilicon has been achieved at lowtemperatures using Si₂H₆ and B₂H₆ as source gases, but there aredisadvantages associated with use of these source gases. B₂H₆ tends todope surfaces nonuniformly. Dopant concentration thus tends to be highlynonuniform across the wafers in a reactor, and even across each wafer.Better uniformity is achieved using B₂H₆ at lower temperatures. However,at lower deposition temperatures, films deposited using Si₂H₆ and B₂H₆are amorphous. Amorphous films require a high temperature anneal to makethe film conductive. Further, Si₂H₆ is less commonly used insemiconductor fabrication than is SiH₄, and thus more expensive and lessreadily available.

[0026] It has largely been assumed that low-temperature deposition of P+polysilicon using SiH₄ and BCl₃ without use of hydrogen plasma anneal orcomplex deposition cycles is impractical. The present applicationdiscloses, however, that by flowing very high amounts of BCl₃ withSiH_(4,) the deposition rate and film quality of P+ polysilicon at lowtemperatures can be significantly improved.

[0027] Briefly, by providing a surface and substantially simultaneouslyflowing SiH₄ and BCl₃ over the surface at a temperature less than orequal to 500 degrees Celsius, under conditions that achieve an averageconcentration in the doped polysilicon film of between about 7×10²⁰ andabout 3×10²¹ boron atoms/cm³, a polysilicon film having a sheetresistance less than about 280 ohms per square can be achieved.Referring again to FIG. 2, it will be seen that at depositiontemperature below 500 degrees, even as low as 460 degrees, the sheetresistance for a film with a boron concentration of 7×10²⁰ atoms/cm³ (asshown on curve B) ranges between about 200 and below about 280ohms/square.

[0028] As FIG. 2 shows, for concentrations of 2×10²⁰ and 7×10²⁰ boronatoms/cm³, sheet resistance generally increases with decreasingdeposition temperature below about 520 degrees Celsius. It is suspectedthat this increase may be due to decreased activation of the dopant.

[0029]FIG. 3 is a graph showing deposition rate of silicon in angstromsper minute at various deposition temperatures. Curve D shows depositionrates for undoped silicon, curve E for in-situ doped silicon with anaverage dopant concentration of 2×10²⁰ boron atoms/cm³, curve F forin-situ doped silicon with an average dopant concentration of 7×10²⁰boron atoms/cm³, and curve G for in-situ doped silicon with an averagedopant concentration of 2×10²¹ boron atoms/cm³. For all curves, thesource gas providing silicon was SiH₄; in the doped cases, the sourcegas providing boron atoms comprised BCl₃. The gases were flowedsubstantially simultaneously. Helium was used to dilute BCl₃ and as amixing gas for SiH₄ and BCl₃.

[0030] It will be seen that at a given temperature, the deposition rategenerally increases as the dopant concentration increases.

[0031] Deposition for the points shown on curves D, E, F, and G tookplace at 400 mTorr. SiH₄ was flowed at 500 standard cubic centimetersper minute (sccm) and helium, an inert gas, at 700 sccm. For thedepositions on curve E, in which the average dopant concentration was2×10²⁰ boron atoms/cm³, 0.5 percent BCl₃ (balance helium) was flowedsubstantially simultaneously in addition to the SiH₄ and helium. For thedepositions on curve F and G, in which the average dopant concentrationswere 7×10²⁰ boron atoms/cm³ and 2×10²¹ boron atoms/cm³ respectively, 1.5percent BCl₃ (balance helium) was flowed substantially simultaneously inaddition to the SiH₄ and helium.

[0032] Preferred embodiments of the present invention will now bedescribed in more detail.

[0033] In a preferred embodiment, a substrate having a surface is placedin a reactor. P+ polysilicon formed according to the present inventionmay be deposited on any material to which it adheres, for examplesilicon dioxide. At temperatures less than or equal to about 500 degreesC., source gases SiH₄ and BCl₃ are substantially simultaneously flowedover the surface under conditions sufficient to achieve in the dopedpolysilicon an average concentration of between about 7×10²⁰ and about3×10²¹ boron atoms/cm³.

[0034] The temperature during deposition is preferably between about 450and about 480 degrees, more preferably between about 460 and 475degrees.

[0035] Many possible flow conditions will produce in-situ dopedpolysilicon of the named dopant concentrations. Pressure is preferablybetween about 200 mTorr and about 1 Torr, more preferably about 400mTorr. Above about 1 Torr, the risk of gas-phase nucleation increases,which will prevent formation of a satisfactory film, though the riskdecreases with lower temperature.

[0036] At least two source gases will be substantially simultaneouslyflowed. For two or more gases to be “substantially simultaneously”flowed means that the gases are flowed at the same time during thedeposition, the flows overlapping. It is not essential that the flows ofboth gases start or end at the same instant. The first source gascomprises SiH₄. Preferably the first source gas is SiH₄. In a preferredembodiment, SiH₄ is flowed at about 400 sccm.

[0037] Using conventional equipment, very low flows of gas can bedifficult to control. The second gas could be pure BCl₃, but to improveuniformity and flow control, the second gas is preferably BCl₃ mixedwith an inert gas, such as helium, argon, or nitrogen; preferablyhelium. The second source gas thus comprises BCl₃. The second source gasmay also comprise an inert gas, which may be helium. The percentage ofBCl₃ in the second source gas may be as low as 0.1 percent. In apreferred embodiment, a mixture of about 1.5 percent BCl₃ and thebalance helium is flowed at about 10 sccm.

[0038] As noted, the flows, temperature, and the concentrations of SiH₄(in the first source gas) and BCl₃ (in the second source gas) should beselected to achieve a concentration of boron atoms in the polysiliconfilm averaging between about 7×10²⁰ and about 3×10²¹ boron atoms/cm³.

[0039] The following table details flow of BCl₃ in sccm used to depositdoped polysilicon at concentrations of 7×10²⁰ boron atoms/cm³ and at2×10²¹ boron atoms/cm³ for different deposition temperatures: Deposition1.5% BCl₃ 1.5% BCl₃ Temperature sccm, ccn = sccm, ccn = degrees C. 7 ×10²⁰/cm³ 2 × 10²¹/cm³ 460 10 20 490 18 36 520 34 68 550 62 124  580 114 228 

[0040] In general, as temperature drops, more boron atoms will beincorporated, so the skilled practitioner will adjust other aspects ofdeposition conditions, including flows, concentrations, and pressures,accordingly, as a matter of routine experimentation.

[0041] In addition to the first source gas comprising SiH₄ and thesecond source gas comprising BCl₃, flowing an inert gas, such as helium,argon, or nitrogen, can improve mixing and lead to more uniform dopantconcentrations across a wafer and across wafers in a reactor. In apreferred embodiment, helium is flowed at about 700 sccm.

[0042] More generally, the concentration of boron atoms in the film iscorrelated with the ratio of SiH₄ to BCl₃. To increase the concentrationof boron atoms in the film, for example, the skilled practitioner willchange the ratio by decreasing SiH₄ (by decreasing concentration orflow) or increasing BCl₃ (by increasing concentration or flow.)

[0043] Preferred embodiments of P+ polysilicon deposited according tothe method of the present invention provide an additional advantage inthat such polysilicon has excellent step coverage. It is common insemiconductor processing to coat uneven surfaces with a deposited layer.If a material has good step coverage, it can be deposited over unevensurfaces, for example over trenches, and will cover horizontal surfacesat the top and bottom of the trenches, and vertical sidewalls oftrenches, with a substantially uniform thickness.

[0044] Poor step coverage can make it difficult or impossible to fill ahigh-aspect ratio trench or via. As shown in FIG. 4a, during chemical orvapor deposition an overhang 16 may form at the opening of the trench.This results in a shadowing effect, sheltering the lower sidewalls andbottom of the via from further deposition and causing a void if the lip“pinches off” and closes.

[0045] As shown in FIG. 4b, in-situ doped polysilicon formed accordingto the present invention can be deposited with good sidewall and bottomcoverage, and without tending to pinch off and form voids if depositedover a trench, by avoiding formation of a significant overhang.Specifically, for a P+ polysilicon film 18 formed according to thepresent invention, at temperatures below about 500 degrees, depositedover a substantially vertical sidewall 20 descending from asubstantially horizontal surface 22, the thickness 24 at its thinnestpoint on sidewall 20 of the film 18 will be at least 80 percent of thethickness 26 of film 18 on the sidewall 20 at the top of the sidewall,when thickness 28 of the film 18 on horizontal surface 22 is at least200 angstroms. “Thickness” here is presumed to be measured perpendicularto the sidewall or surface, whichever is named. If the level of fill isabove the top of the sidewall, then the thickness at the thinnest pointwill be the same as the thickness at the top of the sidewall, unlessthere are any voids; in this case the thinnest point will be measuredfrom the void, as in FIG. 4c. Overhang is undesirable whether or not thevertical sidewall is a trench sidewall.

[0046] In-situ doped polysilicon formed according to the presentinvention can be used in any semiconductor devices or arrays of devices,including memory arrays, that are advantageously formed at lowtemperature. Examples of such devices include, but are not limited to,logic or memory devices, transistors formed in monocrystalline silicon,thin-film transistors (in which polysilicon could form a portion of achannel), capacitors, resistors, inductors, diodes, interconnects,PROMs, EPROMs, and EEPROMs. One such semiconductor device may compriseportions of a diode separated by an antifuse which become a diode whenthe antifuse is ruptured. Arrays of such devices include, but are notlimited to, two dimensional and monolithic three dimensional arrays.

[0047] P+ polysilicon formed according to the present invention could beadvantageously used in monolithic three dimensional memories such asthose described in Johnson et al., U.S. Pat. No. 6,034,882, “Verticallystacked field programmable nonvolatile memory and method offabrication”; Johnson, U.S. Pat. No. 6,525,953, “Vertically stackedfield programmable nonvolatile memory and method of fabrication”; Knallet al., U.S. Pat. No. 6,420,215, “Three Dimensional Memory Array andMethod of Fabrication”; Lee et al., U.S. patent application Ser. No.09/927648, “Dense Arrays and Charge Storage Devices, and Methods forMaking Same,” filed Aug. 13, 2001; and Herner et al., U.S. patentapplication Ser. No. 10/326470, “An Improved Method for MakingHigh-Density Nonvolatile Memory,” filed Dec. 19, 2002, all herebyincorporated by reference. P+ polysilicon formed according to thepresent invention can be used whenever P+ or doped polysilicon is calledfor in any of the patents or applications name herein.

[0048] Monolithic three dimensional memories present particularchallenges for temperature compatibility of processing. In simpler,conventional two dimensional structures (without stacked memory levels),it's possible to combine doped polysilicon, requiring temperatures over550 degrees C., and aluminum, requiring temperatures not exceeding 475degrees C., if the doped polysilicon is formed at higher temperaturesfirst, then aluminum metallization is formed later.

[0049] In contrast, in monolithic three dimensional memories, a memorylevel, including all of its associated metallization, is formed, andthen another memory level is formed above it. This means that allelements of a memory level (including, for example, aluminummetallization) are subjected to the temperatures required to form everyelement (including, for example, low-resistance polysilicon) of the nextlevel. Low-temperature formation of low-resistance polysilicon in onememory level offers the advantage that it does not damage aluminummetallization, or other structures with low temperature tolerances, thatalready exist in a previously-formed level. Thus a low-temperaturemethod to form in-situ doped polysilicon can be particularly useful inmonolithic three-dimensional emories.

EXAMPLE

[0050] In one example, P+ polysilicon according to the present inventionwas formed on silicon wafers in an ASML RVP 9000. The ASML RVP 9000 hasa capacity of 176 wafers. Every other slot was filled, so 88 wafers wereused at an effective pitch of 8.6 mm. Of these 88 wafers, only 75 wereproduct wafers, centered vertically; the rest were dummy wafers. Theproduct wafers had previously had a layer of oxide deposited on them,and P+ polysilicon was deposited on the oxide. The remainder of thewafers were dummy wafers coated with a protective silicon nitride filmto prevent breakage, as described in Herner et al., “Dummy Wafers andMethods for Making the Same,” U.S. patent application Ser. No.10/036291, filed Nov. 7, 2001, hereby incorporated by reference. Thepressure was stabilized at 400 mTorr and the temperature at 460 degreesC. SiH₄ was flowed at 500 sccm, helium was flowed at 700 sccm, and 1.5percent BCl₃ (balance helium) was flowed at 10 sccm to deposit 2000angstroms of P+ polysilicon.

[0051] The foregoing detailed description has described only a few ofthe many forms that this invention can take. For this reason, thisdetailed description is intended by way of illustration, and not by wayof limitation. It is only the following claims, including allequivalents, which are intended to define the scope of this invention.

1-11. (Cancelled).
 12. A semiconductor device comprising in-situ dopedpolysilicon formed by a method comprising: providing a surface whereinaluminum exists at or below the surface; and substantiallysimultaneously flowing SiH₄ and BCl₃ over the surface at a temperatureless than about 500 degrees Celsius, wherein an average concentration ofboron atoms in the polysilicon of between about 7×10²⁰ and about 3×10²¹per cubic centimeter is achieved.
 13. The semiconductor device of claim12 wherein the temperature is between about 450 and about 480 degreesCelsius.
 14. The semiconductor device of claim 13 wherein the device isa thin film transistor.
 15. The semiconductor device of claim 14 whereinthe polysilicon forms a portion of a channel of the thin filmtransistor.
 16. The semiconductor device of claim 13 wherein the devicecomprises a portion of a diode.
 17. The semiconductor device of claim 13wherein the pressure is between about 200 mTrr and about 1 Torr.
 18. Amonolithic three dimensional memory comprising: aluminum; andpolysilicon formed after formation of the aluminum by a methodcomprising: substantially simultaneously flowing SiH₄ and BCl₃ at atemperature less than or equal to about 500 degrees Celsius, wherein anaverage concentration of boron atoms in the polysilicon is between about7×10²⁰ and about 3×10^(2l) per cubic centimeter, wherein the monolithicthree dimensional memory comprises two or more memory levels.
 19. Thememory of claim 18 wherein the temperature is between about 450 andabout 480 degrees Celsius.
 20. The memory of claim 19 wherein thepressure is between about 200 mTorr and about 1 Torr.
 21. The memory ofclaim 20 wherein the BCl₃ is provided in a source gas, and is 1.5percent of the source gas. 22-26. (Cancelled).
 27. A device comprising:aluminum; and an in-situ doped polysilicon film wherein: the polysiliconfilm was deposited at a temperature less than about 475 degrees Celsius;and the polysilicon film is deposited by substantially simultaneouslyflowing a first source gas comprising SiH₄ and a second source gascomprising BCl₃, wherein the film is formed above the aluminum.
 28. Thedevice of claim 27 wherein the polysilicon film is deposited bysubstantially simultaneously flowing a first source gas comprising SiH₄and a second source gas comprising BCl_(3.)
 29. The device of claim 28wherein the temperature is between about 450 and about 480 degreesCelsius.
 30. The device of claim 29 wherein an average concentration ofboron atoms in the polysilicon film is between 7×10²⁰ and 3×10²¹ percubic centimeter.
 31. The device of claim 30 wherein the second sourcegas further comprises an inert gas.
 32. The device of claim 31 whereinthe inert gas is helium.
 33. The device of claim 27 wherein BCl₃ isabout 1.5 percent of the second source gas.
 34. The semiconductor deviceof claim 12 wherein the aluminum is aluminum metallization.
 35. Thememory of claim 18 further comprising aluminum metallization, thealuminum metallization comprising the aluminum.
 36. The device of claim27 wherein the aluminum is aluminum metallizalion.